1. Field of the Invention
This invention relates to the field of voltage regulators, and particularly to methods of improving a voltage regulator's response to a load transient.
2. Description of the Related Art
The purpose of a voltage regulator is to provide a nearly constant output voltage to a load, despite being powered by an unregulated input voltage and having to meet the demands of a varying load current.
In some applications, a regulator is required to maintain a nearly constant output voltage for a step change in load current; i.e., a sudden large increase or decrease in the load current demanded by the load. For example, a microprocessor may have a "power-saving mode" in which unused circuit sections are turned off to reduce current consumption to near zero; when needed, these sections are turned on, requiring the load current to increase to a high value--typically within a few hundred nanoseconds.
When there is a change in load current, some deviation in the regulator's output voltage is practically unavoidable. The magnitude of the deviation is affected by both the capacitance and the equivalent series resistance (ESR) of the output capacitor: a smaller capacitance or a larger ESR increase the deviation. For example, for a switching voltage regulator (which delivers output current via an output inductor and which includes an output capacitor connected in parallel across the load), a change in load current (.DELTA.I.sub.load) results in a change in the regulator's output voltage unless 1) the current delivered to the load instantaneously increases by .DELTA.I.sub.load, or 2) the capacitance of the output capacitor is so large and its ESR is so small that the output voltage deviation would be negligible. The first option is impossible because the current in the output inductor cannot change instantaneously. The time required to accommodate the change in load current can be reduced by reducing the inductance of the output inductor, but that eventually requires increasing the regulator's switching frequency, which is limited by the finite switching speed of the switching transistors and the dissipation in the transistors' driver circuit. The second option is possible, but requires a very large output capacitor which is likely to occupy too much space on a printed circuit board, cost too much, or both.
For applications requiring the regulator's output voltage to meet a narrow load transient response specification, i.e., a specification which narrowly limits the allowable output voltage deviation for a bidirectional step change in load current, this inevitable deviation may be unacceptably large. As used herein, ".DELTA.V.sub.out " refers to a regulator's output voltage deviation specification, as well as to peak-to-peak output voltage deviations shown in graphs. The most obvious solution for improving load transient response is to increase the output capacitance and/or reduce the ESR of the output capacitor. However, as noted above, a larger output capacitor (which provides both more capacitance and lower ESR) requires more volume and more PC board area, and thereby more cost.
One approach to improving load transient response is shown in FIG. 1. A switching voltage regulator 10 includes a push-pull switch 12 connected between a supply voltage V.sub.in and ground, typically implemented with two synchronously switched power MOSFETs 14 and 16. A driver circuit 18 is connected to alternately switch on one or the other of MOSFETs 14 and 16. A duty ratio modulator circuit 20 controls the driver circuit; circuit 20 includes a voltage comparator 22 that compares a sawtooth clock signal received from a clock circuit 24 and an error voltage received from a error signal generating circuit 26. Circuit 26 typically includes a high-gain operational amplifier 28 that receives a reference voltage V.sub.ref at one input and a voltage representative of the output voltage V.sub.out at a second input, and produces an error voltage that varies with the difference between V.sub.out and the desired output voltage. The regulator also includes an output inductor L connected to the junction between MOSFETs 14 and 16, an output capacitor 30, shown represented as a capacitance C in series with an equivalent series resistance R.sub.e, and a resistor R.sub.s connected between the output inductor and the output capacitor. V.sub.out is connected to drive a load 32.
In operation, MOSFETs 14 and 16 are driven to alternately connect inductor L to V.sub.in and ground, with a duty ratio determined by duty ratio modulator circuit 20; the duty ratio varies in accordance with the error voltage produced by error amplifier 28. The current in inductor L flows into the parallel combination of output capacitor 30 and load 32. The impedance of capacitor 30 is much smaller at the switching frequency than that of load 32, so that the capacitor filters out most of the AC components of the inductor current and virtually all of the direct current is delivered to load 32.
Without series resistor R.sub.s, the voltage fed back to circuit 26 is equal to V.sub.out, and the regulator's response to a step change in load current is that of a typical switching regulator; a regulator's output voltage V.sub.out is shown in FIG. 2a for a step change in load current I.sub.load shown in FIG. 2b. Because the current in L cannot change instantaneously, a sudden increase in I.sub.load causes V.sub.out to spike downward; the control loop eventually forces V.sub.out back to a nominal output voltage V.sub.nom. Similarly, when I.sub.load later steps down, V.sub.out spikes up before returning to V.sub.nom. The total deviation in output voltage .DELTA.V.sub.out for a step change in load current is determined by the difference between the two voltage spike peaks. If the regulator is subject to a narrow load transient response specification, this deviation may exceed the tolerance allowed.
Connecting resistor R.sub.s in series with inductor L (at an output terminal 34) can reduce .DELTA.V.sub.out ; one possible response with R.sub.s included is shown in FIG. 3a for a step change in load current shown in FIG. 3b. With R.sub.s in place, the control loop no longer causes V.sub.out to recover to V.sub.nom ; rather, V.sub.out recovers to a voltage given by the voltage at terminal 34 minus the product of .DELTA.I.sub.load and R.sub.s. That is, the steady-state value of V.sub.out for a light load will be higher than it is for a heavy load, by .DELTA.I.sub.load *R.sub.s. Making R.sub.s approximately equal to the ESR of the output capacitor can provide a somewhat narrower .DELTA.V.sub.out than can be achieved without the use of R.sub.s.
One disadvantage of the circuit of FIG. 1 is illustrated in FIGS. 4a and 4b. In this case, the load current (FIG. 4b) steps back down before V.sub.out (FIG. 4a) has settled to a steady-state value. With V.sub.out higher than it was in FIG. 3a at the instant I.sub.load falls, the peak of the upward V.sub.out spike is also higher, making the overall deviation .DELTA.V.sub.out greater than it would otherwise be. This larger deviation means that to satisfy a particular narrow output voltage deviation specification, regulator 10 must use a larger output capacitor that has a proportionally smaller ESR. The cost of a capacitor is approximately inversely proportional to its ESR, so that meeting the specification may be prohibitively expensive.
Another disadvantage of the FIG. 1 circuit is the considerable power dissipation required of series resistor R.sub.s. For example, assuming an R.sub.s of 5 m.OMEGA. and a maximum load current of 14.6 A, the dissipation in R.sub.s will be 1.07 W.
An approach to improving a regulator's load transient response using a different control principle is disclosed in D. Goder and W. R. Pelletier, "V.sup.2 Architecture Provides Ultra-Fast Transient Response in Switch Mode Power Supplies", HFPC Power Conversion, September 1996 Proceedings, pp. 19-23. The regulator described therein includes a push-pull switch, a driver circuit, an error amplifier, and an output inductor and capacitor similar to those shown in FIG. 1. A signal representing the regulator's output voltage is fed to both the error amplifier and to a voltage comparator which also receives the error amplifier's output. When the regulator's output voltage exceeds the output of the error amplifier, the comparator's output goes high and triggers a monostable multivibrator, which turns off the upper switching transistor for a predetermined time interval.
The transient response of this circuit is designed to be faster than that of the circuit in FIG. 1. A load current step immediately changes the voltage at the comparator, bypassing the sluggishness of the error amplifier and thereby shortening the response time. However, even with a shorter response time, the shape of the response trace still resembles that shown in FIG. 3a, with little to no improvement in the magnitude of .DELTA.V.sub.out.
Another switching regulator is described in L. Spaziani, "Fueling the Megaprocessor--a DC/DC Converter Design Review Featuring the UC3886 and UC3910", Unitrode Application Note U-157, pp. 3-541 to 3-570. This regulator employs a control principle known as "average current control", in which regulation is achieved by controlling the average value of the current in the output inductor. A resistor is connected in series with the regulator's output inductor, and a current sense amplifier (CSE) is connected across the resistor to sense the inductor current. The output of the CSE is fed to a current error amplifier along with the output of a voltage error amplifier that compares the regulator's output voltage with a reference voltage. A comparator receives the output of the current error amplifier at one input and a sawtooth clock signal at its other input; the comparator produces a pulse-width modulated output to drive a push-pull switch via a driver circuit.
In operation, an increase in load current causes an output voltage decrease, increasing the error signal from the voltage error amplifier. This increases the output from the current error amplifier, which in turn causes the duty ratio of the pulses produced by the comparator to increase. This increases the current in the output inductor to bring up the output voltage. The voltage error amplifier is configured to provide a non-integrating gain, and this, in combination with average current control, gives the regulator a finite and controllable output resistance. This permits the output voltage to be positioned, similar to the way in which series resistor R.sub.s affected the response of the FIG. 1 circuit. However, as is clearly shown in FIG. 32 of the reference, the obtainable response again resembles that of FIG. 3a, with a .DELTA.V.sub.out that may still exceed a narrow output voltage deviation specification.